Electro-optical devices are structures that have electrical properties that change when contacted with electromagnetic radiation, or optical properties that change with application of an electric field. Examples of electro-optical devices include photodetectors, which can be made from materials that generate secondary, rather than primary photocurrents. In a secondary photoconductor, one of the charge carriers is more mobile than the other and cycles through the circuit amplifying the current until it recombines with the slower carrier. In order to observe secondary photocurrents the mobile carrier must be able to be injected from electrodes into the photoconductor.